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Thursday, February 19, 2009

Windows Presentation Foundation

he architecture of Windows Presentation Foundation spans across both managed code and native code components. However, the public API exposed is only available via managed code. While the majority of WPF is in managed code, the composition engine which renders the WPF applications is a native component. It is named Media Integration Layer (MIL) and resides in milcore.dll. It interfaces directly with DirectX and provides basic support for 2D and 3D surfaces, timer-controlled manipulation of contents of a surface with a view to exposing animation constructs at a higher level, and compositing the individual elements of a WPF application into a final 3D "scene" that represents the UI of the application and renders it to the screen.[5][6] The media codecs are also implemented in unmanaged code, and are shipped as windowscodecs.dll.[5] In the managed world, PresentationCore (presentationcore.dll) provides a managed wrapper for MIL and implements the core services for WPF,[5] including a property system that is aware of the dependencies between the setters and consumers of the property, a message dispatching system by means of a Dispatcher object to implement a specialized event system and services which can implement a layout system such as measurement for UI elements.[6] PresentationFramework (presentationframework.dll) implements the end-user presentational features, including layouts, time-dependent, story-board based animations, and data binding.[6]

WPF exposes a property system for objects which inherit from DependencyObject, that is aware of the dependencies between the consumers of the property, and can trigger actions based on changes in properties. Properties can be either hard coded values or expressions, which are specific expressions that evaluate to a result. In the initial release, however, the set of expressions supported is closed.[6] The value of the properties can be inherited from parent objects as well. WPF properties support change notifications, which invoke bound behaviors whenever some property of some element is changed. Custom behaviors can be used to propagate a property change notification across a set of WPF objects. This is used by the layout system to trigger a recalculation of the layout on property-changes, thus exposing a declarative programming style for WPF, whereby almost everything, from setting colors and positions to animating elements can be achieved by setting properties.[6] This allows WPF applications to be written in XAML, which is a declarative mark-up language, by binding the keywords and attributes directly to WPF classes and properties.

The UI elements of an WPF application are maintained as a class of Visual objects. Visual objects provide a managed interface to a composition tree which is maintained by Media Integration Layer (MIL). Each element of WPF creates and adds one or more composition nodes to the tree. The composition nodes contain rendering instructions, such as clipping and transformation instructions, along with other visual attributes. Thus the entire application is represented as a collection of composition nodes, which are stored in a buffer in the system memory. Periodically, MIL walks the tree and executes the rendering instructions in each node, thus compositing each element on to a DirectX surface, which is then rendered on screen. MIL uses the painter's algorithm, where all the components are rendered from back of the screen to the front, which allows complex effects like transparencies to be easily achieved. This rendering process is hardware accelerated using the GPU.[6] The composition tree is cached by MIL, creating a retained mode graphics, so that any changes to the composition tree needs only to be incrementally communicated to MIL. This also frees the applications of managing repainting the screen, MIL can do that itself as it has all the information necessary. Animations can be implemented as time-triggered changes to the composition tree. On the user visible side, animations are specified declaratively, by setting some animation effect to some element via a property and specifying the duration. The code-behind updates the specific nodes of the tree, via Visual objects, to represent both the intermediate states at specified time intervals as well as the final state of the element. MIL will render the changes to the element automatically.

All WPF applications start with two threads: one for managing the UI and another background thread for handling rendering and repainting.[7] Rendering and repainting is managed by WPF itself, without any developer intervention. The UI thread houses the Dispatcher (via an instance of DispatcherObject), which maintains a queue of UI operations that need to be performed (as a tree of Visual objects), sorted by priority. UI events, including changing a property that affects the layout, and user interaction events raised are queued up in the dispatcher, which invokes the handlers for the events. Microsoft recommends that the event handlers only update the properties to reflect new content for application responsiveness; the new content be generated or retrieved in a background thread.[7] The render thread picks up a copy of the visual tree and walks the tree calculating which components will be visible and renders them to Direct3D surfaces. The render thread also caches the visual tree, so only changes to the tree need to be communicated, which will result in updating only the changed pixels. WPF supports an extensible layout model. Layout is divided into two phases: Measure and Arrange. The Measure phase recursively calls all elements and determine the size they will take. In the Arrange phase, the child elements are recursively arranged by their parents, invoking the layout algorithm of the layout module in use.[6][8]

Features

The following is a partial list of WPF features.

Graphical Services

* All graphics (including desktop items like windows) are Direct3D applications. o This aims to provide a unified avenue for displaying graphics, as well as more advanced graphical features. o Routing the graphics through Direct3D allows Windows to offload some graphics tasks to the Graphics Processing Unit found on the computer's graphics card. This can reduce the workload on the computer's Central processing unit. o Supports vector-based graphics, which allow lossless scaling. o Supports 3D model rendering and interaction in 2D applications. o Interactive 2D content can be overlaid on 3D surfaces, natively.[9]

Interoperability

* WPF provides interoperability with the Windows API: Via hosting, one can use Windows Presentation Foundation inside existing Win32 code, or one can use existing Win32 code inside Windows Presentation Foundation.[10] * Interoperability with Windows Forms is also possible through the use of the ElementHost and WindowsFormsHost classes.

Media Services

* WPF provides shape primitives for 2D graphics along with a built-in set of brushes, pens, geometries, and transforms. * The 3D capabilities in WPF are a subset of the full feature set provided by Direct3D. However, WPF provides tighter integration with other features like user interface (UI), documents, and media. This makes it possible to have 3D UI, 3D documents, and 3D media. * There is support for most common image formats. * WPF supports the video formats WMV, MPEG and some AVI files by default, but since it is Windows Media Player running beneath, WPF can use all the codecs installed for it. * WPF supports time-based animations, in contrast to the frame-based approach. This decouples the speed of the animation from how the system is performing.

Animations

* Time-based animation. Scene redraws are time triggered.[11] * Presentation timers are initialized and managed by WPF.[11] * Scene changes co-ordinated by using a storyboard.[11]. An example of animating a storyboard from C# is here. * Animations can be triggered by other external events, including user action.[11] * Animation effects can be defined on a per-object basis, which can be accessed directly from XAML markup.[11] * Ships with a set of predefined effects, such as fade out for all controls.[11]

Data binding

* WPF has a built-in set of data services to enable application developers to bind and manipulate data within applications. There exists support for three types of data binding: o one time: where the client ignores updates on the server. o one way: where the client has read-only access to data. o two way: where client can read from and write data to the server. * LINQ queries, specifically LINQ to XML, can also act as data sources for data binding.[9] * Binding of data has no bearing on its presentation. WPF provides data templates to control presentation of data.

* A set of built-in controls is provided as part of WPF, containing items such as button, menu, grids and list box. * A powerful concept in the WPF is the logical separation of a control from its appearance. o A control's template can be overridden to completely change its visual appearance. o A control can contain any other control or layout, allowing for unprecedented control over composition. * Features retained mode graphics, so that applications do not have to be bothered with repainting the display.

* Annotations can be applied on a per-object basis, for objects in a Document or FlowDocument. * WPF only provides the capability for creating, storing and managing annotations; each application must expose the UI on their own.

* WPF provides for bitmap effects, however, they are rendered in hardware only in .NET Framework 3.5 SP1 or newer. GPU features like pixel shaders are not used for bitmap effects. * Special effects such as dropshadows and blurring are built in.

* WPF natively supports paginated documents. It provides the DocumentReader class, which is for reading fixed layout documents. The FlowDocumentReader class offers different view modes such as per-page or scrollable and also reflows text if the viewing area is re sized. * Natively supports XML Paper Specification documents. * Supports reading and writing paginated documents using Open Packaging Convention.

WPF includes a number of typographic and text rendering features that were not available in GDI. This is the first Windows programming interface to expose OpenType features to software developers, supporting both OpenType, TrueType, and OpenType CFF (Compact Font Format) fonts.

WPF handles texts in Unicode, and handles texts independent of global settings, such as system locale. In addition, fallback mechanisms are provided to allow writing direction (horizontal versus vertical) handled independent of font name; building international fonts from composite fonts, using a group of single-language fonts; composite fonts embedding. Font linking and font fallback information is stored in a portable XML file, using composite font technology.[12] The XML file has extension .CompositeFont.

The WPF text engine also supports built-in spell checking. It also supports such features as automatic line spacing, enhanced international text, language-guided line breaking, hyphenation, and justification, bitmap effects, transforms, and text effects such as shadows, blur, glow, rotation etc. Animated text is also supported; this refers to animated glyphs, as well as real-time changes in position, size, color, and opacity of the text.

WPF text rendering takes advantage of advances in ClearType technology, such as sub-pixel positioning, natural advance widths, Y-direction anti-aliasing, hardware-accelerated text rendering, as well as aggressive caching of pre-rendered text in video memory.[13] ClearType cannot be turned off in WPF applications.[14] Unlike the ClearType in GDI or GDI+, WPF ClearType does not snap glyphs to pixels horizontally, leading to a loss of contrast disliked by some users. [15]

The extent to which glyphs are cached is dependent on the video card. DirectX 10 cards are able to cache the font glyphs in video memory, then perform the composition (assembling of character glyphs in the correct order, with the correct spacing), alpha-blending (application of anti-aliasing), and RGB blending (ClearType's sub-pixel color calculations), entirely in hardware. This means that only the original glyphs need to be stored in video memory once per font (Microsoft estimates that this would require 2 MB of video memory per font), and other operations such as the display of anti-aliased text on top of other graphics—including video—can also be done with no computation effort on the part of the CPU. DirectX 9 cards are only able to cache the alpha-blended glyphs in memory, thus requiring the CPU to handle glyph composition and alpha-blending before passing this to the video card. Caching these partially-rendered glyphs requires significantly more memory (Microsoft estimates 5 MB per process). Cards that don't support DirectX 9 have no hardware-accelerated text rendering capabilities.

Following the success of markup languages for web development, WPF introduces a new language known as eXtensible Application Markup Language (XAML) (pronounced as "Zammel"), which is based on XML. XAML is designed as a more efficient method of developing application user interfaces[citation needed].

The specific advantage that XAML brings to WPF is that XAML is a completely declarative language. In a declarative programming language, the developer (or designer) describes the behavior and integration of components without the use of procedural programming. This allows someone with little or no traditional programming experience to create an entire working application with no programming. Although it is rare that an entire application will be built completely in XAML, the introduction of XAML allows application designers to more effectively contribute to the application development cycle. Using XAML to develop user interfaces also allows for separation of model and view; which is considered a good architectural principle. In XAML, elements and attributes map to classes and properties in the underlying APIs.

As in web development, both layouts and specific themes are well suited to markup, but XAML is not required for either. Indeed, all elements of WPF may be coded in a .NET language (C#, VB.NET). The XAML code can ultimately be compiled into a managed assembly in the same way all .NET languages are, which means that the use of XAML for development does not incur a performance cost. XAML can also be compiled and run "on demand" similar to an HTML web-page.

Although XAML has been introduced as an integral part of WPF, the XAML standard itself is not specific to WPF (or even .NET). XAML can also be used to develop applications using any programming API and is in itself language independent. Nevertheless, special care has been taken in developing the WPF APIs to maximize interoperability with the declarative model that XAML introduces.

It is likely that many applications, such as Microsoft PowerPoint and Word, will support exporting their content to XAML.

There are also profiles specific to Workflow Foundation, and other domain specific subsets will likely emerge in the future.

Tools

There are a number of development tools available for developing Windows Presentation Foundation applications, most currently of beta quality, some commercially released.

* Microsoft Cider is the XAML designer in the form of an add-in for the Visual Studio 2005 IDE for building Windows Presentation Foundation applications. It is available in CTP form for Visual Studio 2005, there are no plans for an RTM version for Visual Studio 2005. Cider is integrated into Visual Studio 2008.[16] * Microsoft Expression Blend is a designer-oriented tool that provides a canvas for the creation of WPF applications with 2D and 3D graphics, text and forms content. It generates XAML that may be exported into other tools. * Microsoft Expression Design is a bitmap and 2D-vector graphics tool that allows export to XAML. * XAMLPad is a lightweight tool included in the .NET Framework SDK. It can create and render XAML files using a split screen UI layout. It also provides a tree view of the markup in a panel. An improved version, called XamlPadX v3, can be downloaded from the author's blog. Deployment

* WPF isn't just for building traditional standalone applications. Its deployment model offers both standalone and XAML Browser Applications (XBAP) flavors. The programming model for building either flavor of application is similar. o Standalone applications are those that have been locally installed on the computer using software such as ClickOnce or Windows Installer (MSI) and which run on the desktop. Standalone applications are considered full trust and have full access to a computer's resources. o XAML Browser Applications (XBAPs) are programs that are hosted inside a web browser. Hosted applications run in a partial trust sandbox environment, and are not given full access to the computer's resources and not all WPF functionality is available. The hosted environment is intended to protect the computer from malicious applications. Starting an XBAP (pronounced "ex-bap") from an HTML page or vice versa is seamless (there is no security or installation prompt). Although one gets the perception of the application running in the browser, it actually runs in an out-of-process executable different from the browser. As of the release of .NET Framework 3.0, XBAPs only run in Internet Explorer. With the release of .NET Framework 3.5 SP1 they also run in Mozilla Firefox using the included XBAP extension.